This section is intended to introduce the reader to the diverse aspects of the art, which may be related to diverse aspects of the present invention which are described and/or claimed hereinbelow. This discussion is considered to be useful in order to provide the reader with background information so as to facilitate a better understanding of the various aspects of the present invention. Consequently, it must be understood that these statements must be read in this light, and not as a declaration of the prior art.
Any ophthalmic lens, intended to be carried in a frame, is associated with a prescription. Ophthalmically speaking the prescription can comprise a power prescription, positive or negative, as well as an astigmatism prescription. These prescriptions correspond to corrections to be afforded the wearer of the lenses in order to correct the defects of his vision. A lens is mounted in the frame as a function of the prescription and of the position of the wearer's eyes with respect to the frame.
In the simplest cases, the prescription reduces to a power prescription, positive or negative, and optionally an astigmatism prescription, the lens is then termed “unifocal”. When the prescription does not comprise any astigmatism, the lens exhibits symmetry of revolution. It is simply mounted in the frame so that the wearer's principal direction of gaze coincides with the axis of symmetry of the lens. For presbyopic wearers, the value of the power correction is different in far vision and in near vision, on account of the difficulties of accommodation in near vision. The prescription is then composed of a far vision power value and of an addition (or power progression) representative of the power increment between far vision and near vision; this amounts to a power prescription for far vision and to a power prescription for near vision. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in EP 2 249 195 B1 or EP 2 251 733 B1. Progressive multifocal ophthalmic lenses comprise a far vision zone, a near vision zone, an intermediate vision zone, a principal meridian of progression passing through these three zones. Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition, which corresponds to the variation of power between the far vision zone and the near vision zone. More precisely, the addition, denoted A, corresponds to the power variation on the meridian between a point VL of the far vision zone and a point VP of the near vision zone, which are respectively called the far vision reference point and the near vision reference point, and which represent the points of intersection of the gaze and of the surface of the lens for vision at infinity and for reading vision.
In a conventional manner, a lens can be defined by its base (or mean sphere of its front face in far vision) and by a power addition in the case of a multifocal lens. On the basis of semi-finished lenses, only one face of which complies with a given addition/base pair, it is possible to prepare lenses suited to each wearer, by simple machining of a prescription face which is generally spherical or toric.
For any ophthalmic lens, the laws of optics for the paths of rays give rise to the appearance of optical defects when the light rays deviate from the central axis of the lens. These known defects which among others comprise a power defect and an astigmatism defect can in a generic manner be called “obliquity defects of the rays”. The person skilled in the art knows how to compensate these defects partially. For example, EP-A-0 990 939 proposes a method for determining by optimization an ophthalmic lens for a wearer having an astigmatism prescription. Obliquity defects have also been identified for progressive multifocal lenses. For example, WO-A-98 12590 describes a procedure for determining by optimization a suite of multifocal ophthalmic lenses.
An ophthalmic lens comprises an optically useful central zone, which can extend over the entire lens. The expression “optically useful” zone is intended to mean a zone in which the power defect and astigmatism defect have been minimized so as to allow satisfactory visual comfort for the wearer. In a progressive lens, the optically useful central zone will cover at least the far vision zone, the near vision zone and the progression zone.
Generally, the optically useful zone covers the entire lens which exhibits a diameter of restricted value. However, in certain cases, a “peripheral” zone is envisaged around the perimeter of the ophthalmic lens. This zone is termed “peripheral” since it does not meet the prescribed conditions of optical correction and exhibits significant obliquity defects. The optical defects of the “peripheral” zone are not detrimental to the wearer's visual comfort since this zone is situated outside the wearer's customary field of vision. A connection must then be provided between the optically useful central zone and the peripheral zone.
There are principally two situations in which an ophthalmic lens exhibits a peripheral zone such as this. On the one hand, when the lens exhibits a significant diameter which may be imposed by the shape of the frame, for example an elongate frame with a large wrap, and on the other hand when the power prescription is high, the lens then exhibiting a significant edge thickness or center thickness that it is sought to reduce.
In the case of an ophthalmic lens intended to be fitted into a wraparound frame, for example to 15 degrees, the lens exhibits a spherical or toric front face of large camber (or large base), between 6 diopters (denoted “D” in what follows) and 10 diopters, and a rear face calculated specifically to attain the optimal correction of the wearer's ametropia at the optical center and in the field of vision. For example, for a front face having a given curvature, the rear face is machined to ensure the correction as a function of each wearer's ametropia.
The large camber of the front face gives rise to a large thickness of the lens on the edges in the case of a lens with negative power or a large thickness of the lens at the center in the case of a lens with positive power. These large thicknesses increase the weight of the lenses, which is detrimental to the comfort of the wearer and make them unesthetic. Moreover, for certain frames, the edge thickness must be controlled to allow the lens to be mounted in the frame.
For negative lenses, the thicknesses at the edges can be reduced by filing down by virtue of a manual facet. A thinning of the lens can also be controlled by optical optimization. An aspherization or an atorization can be calculated, at least for one of the faces of the lens, by taking into account the wearing conditions of the lens with respect to a lens of small camber of like prescription, so as to decrease the center and edge thicknesses of the lens of large camber. Optical aspherization or atorization solutions such as these are for example described in the documents U.S. Pat. No. 6,698,884, U.S. Pat. No. 6,454,408, U.S. Pat. No. 6,334,681, U.S. Pat. No. 6,364,481 or else WO-A-97 35224.
Moreover, in the case of a lens with strong prescription, the cribbed lens exhibits a significant thickness at the edge, nasal side for a positive lens (case of a hypermetropic wearer) and temporal side for a negative lens (case of a myopic wearer). These edge thickenings complicate the mounting of the lens in the frame and make the ophthalmic lenses heavier to wear.
EP 2 028 529 A1 describes a method for determining the two faces of an ophthalmic lens by taking into account the prescription, information on the position of the lens with respect to the wearer's eyes and information on the geometry of the frame in which the lenses must be mounted. This first solution of the prior art proposes to adapt the curvature of the front face of an ophthalmic lens to improve the esthetics of the mounting of the lens in the frame.
WO 2008/037892 describes a method for determining an ophthalmic lens comprising an optically useful zone, a peripheral zone, such as described above, making it possible to reduce the edge and/or center thickness of the lens as well as a connection zone possessing a curvature profile which is optimized for the comfort of the wearer. This solution of the prior art proposes to reduce the edge and/or center thickness of the ophthalmic lenses by aspherizing or by locally atorizing one face of the lens knowing the other face so as to facilitate mounting in spectacles frames. The proposed solution exhibits a connection between the optically useful central zone and the thinned peripheral zone. Adherence to the prescription reduces to the optically useful zone. This solution is not accepted by all wearers because of the “peripheral” zone in which the vision is not corrected.
The solutions proposed in the prior art do not make it possible to simultaneously solve the problems posed by the esthetics of the mounting of an ophthalmic lens in the frame and those posed by mounting in frames.
Moreover, the need always exists for a lens which better satisfies wearers with optimal optical performance while exhibiting a reduced thickness so as to improve the esthetic aspect of the lens and the wearer's comfort.